Lighning damage, looking to prevent next time

Greetings, I have just joined this forum as you guys and gals have more experience than many of solar PV, and maybe you can advise in this case.

I was just called to help a friend whose panels stopped generating. On investigation, the inverter was fine but a great many of the panel diodes had gone short-circuit. I'm looking for thoughts on the kind of protection that might be added to prevent further outages. The original installer seems to have no idea at all about PV beyond how to fit them together.

The panels are mounted on the ground near sea level in a mountainous area of North Wales, UK. It is a 4kWhp system of 16 x 250w panels. Each panel is wired in three sections with a 15SQ045 diode across each section in the connecting block on the back. The panels were installed as a single string, linked to an inverter nearby. The panels are clamped to aluminium bars attached to a galvanised support structure, concreted into slightly marshy ground. Connecting cables to the invertor are buried.

An 11kV power line runs overhead quite close to the panels (maybe 25 feet above).

The fault turned out to be that

- the six panels on the positive end of the string were in good order with no faults

- the seventh panel had a burned out diode on the positive-side section. The plastic box around the diode had melted and the diode itself had charred and cracked. The diode and the one next to it had also de-soldered. Clearly a major event had hit - the surge current tolerance of a 15SQ045 is 275 Amps.

- all the diodes in the panels from the blown diode to the negative end of the string had gone short-circuit but appeared otherwise undamaged.

- despite all this, there was no visible sign of a strike on the panels or the frames and supports (the diodes are beneath the panels)

Roughly around the time of the failure the electricity supply company experienced a trip on the 11kV system.

I replaced all the diodes that had blown and the system started up fine so there seems to have been no damage to the panels themselves.

My working assumption for the moment was that a strike somewhere on the 11kV system as it runs through the mountains had caused a secondary strike from the 11kV system to the panels below and found its way to ground via the negative side of the string.

Is this likely given the lack of visible damage to the panel at the presumed point of strike? Any other thoughts as to cause?

I have already advised thorough bonding of the structure to earth rods, as currently it relies on the concreted bases and I don't think that is ideal. Any other protection that should be provided?

My friend was all set to buy new panels so it was nice to fix them for a few dollars. Express postage from China cost more than the diodes themselves!

Comments

  • vtmapsvtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: Lighning damage, looking to prevent next time

    Welcome to the forum.

    Its possible that both you and the 11 kv were casualties of a lightning strike... your diodes could fry because of very high induced currents in the wiring. Sure, the 11 kv line may have brought a distant lightning strike to you... or maybe not. Lightning, to my limited understanding, is about as predictable as the weather in the mountains.

    More importantly, is there anything you can do to prevent it happening again? We need many more details about your system and its grounding if we are to give you any advice on that. How far are the panels from the power center? What sort of grounding do you have at each end of that run? What type of conduit (metallic?) , and what's in it? Do you have surge arresters?

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BB.BB. Super Moderators, Administrators Posts: 32,608 admin
    Re: Lighning damage, looking to prevent next time

    Do you have shading from the 11 kV line or from other near by objects (vent pipes, structure, trees, etc.) during the middle of the day... The bypass diodes are there to carry current around the shaded sections of the panel/array. It sounds like the diodes could have overheated from the bypass current.

    My concern is that this is a weakness of many solar panels (bypass diodes that can overheat if carrying full rated current). Diodes get hot and solar panels do not have good heat sinking for the diodes (installed in plastic box behind glass in hot sun).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Lighning damage, looking to prevent next time

    in my opinion there isn't a proper grounding of the array if only the metal supports are just sunk into concrete even if the ground is marshy. each frame should be grounded with a proper ground wire and you can do a series run for each say group of 4 pvs to be run to a single proper 8ft ground rod and be sure to use thick enough bare copper wire. here it would be a minimum of #8 and i don't have the conversions handy for you. you can use more rods if you tie them together with extra heavy bare copper wire and bury it underground.

    now you could attach a good surge suppressor to the array such as those offered up by midnite solar.
    http://www.solar-electric.com/suprde.html
    these do require a good proper ground to work properly.

    anyway this may not have stopped a full blow-out depending on exactly what transpired for if a full lightning stroke did enter that segment of the array almost certainly all bets are off no matter what you do. now in spite of there being no gurantees in a full strike one other possible measure might be to have a lightning rod positioned near the pvs, but at a greater height to be more apt to pass a strike rather than entering the array. of course this needs to be in a position so as to not shade the pvs ever so it would be on the northern side of the array.

    doing all of this after replacing what you must to get the array functional again may give you enough protection to avoid this again, but there's always a chance of it blowing out no matter what you do. all you can do is increase your odds.
  • geoff07geoff07 Registered Users Posts: 2
    Re: Lighning damage, looking to prevent next time

    Thanks for the replies guys. I think the summary is:

    - lightning is a black art so not easy to predict behaviour
    - the grounding of the array isn't up to the minumum required
    - some form of surge suppression is called for.

    The priority is the grounding so I will suggest that that gets done quickly. I will also see what surge suppression is available locally.
  • BB.BB. Super Moderators, Administrators Posts: 32,608 admin
    Re: Lighning damage, looking to prevent next time

    When looking at suppressors for you power (and DC solar) lines--There are different types and many (most/all?) will need replacement if there is a "signifcant" suppression event:
    BB. wrote: »
    I am not sure a MOV is a great idea anyway... Typically, whatever they are in gets blasted with flaming MOV if there is a large surge.

    Placing a MOV in the panel and have it pop--The panel/rear of panel is probably ruined.

    The better point may be the combiner box--But then we have to discuss where the surge is coming from that is being protected... If the lightning or power line cross hits the panels, the MOVs would probably help limit the surge current to the PV Charge Controller or GT inverter. In that case, I probably would suggest that the combiner box be mounted closer to the charger/gt inverter vs right at the panels themselves.

    The only way, I would guess, that the panels could be protected by MOVs would be if the surge was coming from the charge controller/GT inverter--Both relatively unlikely sources of surge current (if everything is working well). Of course, there is the possibility of lightning strike on the conduit/solar PV power lines... But then, you would probably want MOVs at near the panels and near the controllers/inverters (two places).

    Here is a nice photo of a blown MOV:

    http://www.flickr.com/photos/lungstruck/510116147/

    Attachment not found.

    -Bill


    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • YehoshuaAgapaoYehoshuaAgapao Solar Expert Posts: 280 ✭✭
    Re: Lighning damage, looking to prevent next time
    geoff07 wrote: »
    Thanks for the replies guys. I think the summary is:

    - lightning is a black art so not easy to predict behaviour
    - the grounding of the array isn't up to the minumum required
    - some form of surge suppression is called for.

    The priority is the grounding so I will suggest that that gets done quickly. I will also see what surge suppression is available locally.

    My panels are grounded to the rails with a WEEB at every mid-clamp and the #6 bare ground wire is attached to each rail with WEEBLugs (I currently have 10 rails so there are 10 ground connections). The #6 bare copper run continuous to through a ground bar and into two ground rods (soil in AZ is very dry). The ground wires from the combiner boxes go to the ground bar to connect them to the ground rods. The grounds from the PDP goes direct to the 2nd ground rod (the 2nd ground rod has 2 connections the cross-bond, and the ground from the PDP). There is a single Midnite Solar lightning arrestor in each of the combiner boxes. There is a lightning arrestor on the DC positive and DC negative busses. There is also a lighting arrestor on the AC neutral bus, and the generator breaker. There are two lighting arrestors in the main service panel (there wasn't room in the PDP). There is also a lighting arrestor on the generator where the #10 SOOW is spliced to #6 because the generator is about 80 feet away and its an expensive inverter-generator. Not sure if puttting a lighting arrestor on the AC netural bus was a waste, but most home surge protectors have a MOV on L-G, N-G, and L-N. Midnite Solar Lightning arrestors are actually MOV-based type 1 SPDs (surge protection device).
Sign In or Register to comment.